Re-metallized aluminum bond pad, and method for making the same

ABSTRACT

A electroless plating method re-metallizes aluminum bond pads so that the re-metallized bond pads include layers of aluminum, zinc, nickel, and gold. The re-metallized bond pads are wire-bondable and solder wettable, and therefore can be flip-chip bonded. Applications include the realization of hybrid smart pixel arrays for optical interconnections, where an optical transmitter and optical detector are flip-chip bonded directly to respective CMOS driver chips.

This application claims the benefit of U.S. Provisional Application No.60/141,662, filed on Jun. 30, 1999.

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of NSF EEC9520255 awarded by NSF.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is generally related to semiconductor processing. Moreparticularly, the invention is related to re-metallizing aluminum bondpads on a semiconductor integrated circuit so that the re-metallizedbond pads are solder-wettable.

2. Related Art

Cost-effective integration and packaging are the keys to the successfulcommercialization of electronic and optoelectronic components inlarge-volume markets.

Wire-bonding is one well known technique for forming an electricalconnection between two semiconductor integrated circuits (ICs). Duringwire-bonding, metal electrodes (called bond pads) on the respectivesemiconductor ICs are electrically connected together using very smalldiameter wires (called bond wires) that are typically made of gold.Example diameters of bond wires are approximately 20-25 um. Since thebond wires are very thin, the length of the bond wires should be keptrelatively short in order to prevent parasitic reactances from affectingthe circuit performance. Additionally, the bond wires are fragile andcannot be relied on to provide a mechanically rigid connection betweenthe semiconductor ICs.

Another well known integration technique is called “flip-chip” bonding.In flip-chip bonding, two semiconductor ICs having matching bond padpatterns, are soldered together by re-flowing solder bumps that areattached to the bond pads of one of the ICs. During flip-chip bonding,one of the ICs is “flipped” upside down and aligned with the matchingbond pad pattern on the second IC. After which, the solder bumps arere-flowed to perfect the electrical and mechanical connection betweenthe matching bond pad patterns on the two semiconductor ICs.

Flip-chip bonding has multiple advantages over wire-bonding, assumingthere are matching bond pad patterns on the semiconductor ICs. Forexample, flip-chip bonding takes advantage of self-aligning propertiesof solder, which can compensate for IC misalignment up to a few microns.Additionally, all of the solder connections are re-flowedsimultaneously, instead of individually. Additionally, flip chip bondingprovides a rigid mechanical connection between the two semiconductorICs. Finally, flip-chip bonding eliminates the parasitics associatedwith the long bond-wires that are used in wire-bonding.

Despite the advantages of flip-chip bonding, most silicon CMOS chipsthat are produced by commercial foundries have bond pads that are madeof sputtered aluminum alloys, such as Al/Si/Cu(98%, 1%, 1%). Thesealuminum alloys are not solder-wettable, and therefore cannot beflip-chip bonded without modification.

Therefore, what is needed is a process for re-metallizing aluminum alloybond pads so that the bond pads are solder-wettable, and can beflip-chip bonded.

SUMMARY OF THE INVENTION

The present invention is directed to a re-metallized aluminum bond padon a semiconductor integrated circuit (IC), and a method or process ofmaking the same. The re-metallized bond pad includes the followinglayers: the original aluminum layer, a zinc layer, a nickel layer, and agold layer. The re-metallized bond pad is both wire-bondable andsolderable, and can be flip-chip bonded.

The re-metallization process is an electroless plating process thatre-metallizes the aluminum pads with gold. The process works selectivelyon the aluminum pads only, without being detrimental to the underlyingsilicon circuitry that is typically protected by a passivation layer.

The first step of the re-metallization process is to pre-clean thealuminum bond pad to remove dust and wafer processing residue. Thepre-cleaning can be done with organic solvents, including TCA, Acetone,and Methanol.

The next step of the re-metallization process is to de-oxidize thealuminum bond pad to remove the native oxide layer. This can be done bymicro-etching the aluminum bond pad in an acid solution. Theconcentration and immersion time can be varied to adjust the roughnessof the resulting de-oxidized aluminum surface.

The next step in the re-metallization process is to deposit a layer ofzinc on the de-oxidized aluminum using an alkaline zincate solution.Preferably, the zinc layer is applied in two zincate treatments, called“double zincation.” A first seed layer is applied immediately afterde-oxidizing the aluminum bond pad, to prevent the aluminum bond padfrom re-oxidizing. The second zinc treatment is performed after“desmutting” the IC in nitric acid. Double zincation is preferred oversingle zincation, because the intermediate de-smutting step strips thegranulated initial zinc deposit, and produces a more uniform zinc filmover the surface of the bond pad. Additionally, superior uniformity insurface coverage and grain size has been achieved by raising thetemperature of the zincate solution to a range of 38-42 degrees C.during immersion. In contrast, the manufacturer of the zincate solutionsuggests a temperature of 25 C, or room temperature.

The next step in the re-metallization process is to deposit a layer ofnickel onto the zinc layer using an electroless process. The nickellayer seals the aluminum surface as a solder diffusion barrier layer,and also provides hardness, mechanical strength, and solderability tothe bond pad.

The final step in the re-metallization process is to deposit a layer ofgold onto the nickel layer using an immersion process and/or anautocatalytic process. The gold layer protects the re-metallized bondpad from oxidation, and improves solderability and wire-bondability.

The re-metallizing process is a low-cost and efficient technique forre-metallizing aluminum bond pads. The process is low cost because theelectroless plating solutions that are utilized are readily available inthe commercial market. Additionally, no electric current source isnecessary because all the solutions and steps are electroless.Additionally, the number of steps in the process cycle have beenminimized by selecting advantageous combinations of chemical reagents.Potentially hazardous steps, like cyanide zinc pre-treatment and cyanideautocatalytic gold plating are replaced with benign alternativeprocesses.

Another advantage of the invention is that the re-metallizedaluminum/gold bond pads are still wire-bondable. In other words, there-metallized bond pads are suitable for both flip-chip solder bonding,and manual wire-bonding applications. This results in cost and timesavings during hybrid assembly. Additionally, no additional masking orlithographic processing is utilized to protect the bond pads in order toinsure wire-bondability.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates an optical interconnect containing a flip-chip bondedpair including an optical transmitter and receiver, according toembodiments of the invention;

FIG. 2a is a diagram of a 4×4 VCSEL die-substrate assembly with a CMOSdriver chip that is flip-chip solder bonded and wire-bonded, accordingto embodiments of the invention;

FIG. 2b is a diagram of a cross-sectional view of the flip-chip bondedVCSEL die and CMOS driver chip, according to embodiments of theinvention;

FIG. 3 illustrates a flowchart of the complete electrolessre-metallization process, according to embodiments of the presentinvention;

FIG. 4a illustrates a Scanning Electron Microscope (SEM) image of ahighly granulated and non-uniform zinc deposit (magnification 760×),resulting from the improper removal of the native oxide layer that isassociated with aluminum bond pads;

FIG. 4b illustrates a SEM image of a zincated aluminum bond pad(magnification 1,500×), where the oxide layer was properly removed priorto zincation, according to embodiments of the invention;

FIG. 5 illustrates a SEM image of bond pad after electroless nickelplating (magnification 1,500×), according to embodiments of theinvention;

FIG. 6 illustrates a SEM image of bond pad after immersion gold plating(magnification 1,500×), according to embodiments of the invention;

FIG. 7 illustrates a cross section diagram of a electrolesslyre-metallized bond pad showing the different layers of re-metallizationon aluminum, according to embodiments of the present invention;

FIG. 8 illustrates a comparison of the variation of surface roughnessduring the electroless re-metallization process on chemically purealuminum and Al/Si/Cu alloy, according to embodiments of the presentinvention;

FIG. 9 illustrates a diagram of the lateral spreading of re-metallizedbond pads having a mushroom shape on a CMOS chip, according toembodiments of the present invention;

FIG. 10. illustrates a profile of temperature variation during a“tack-and-reflow” process used for flip-chip bonding, according toembodiments of the present invention;

FIG. 11 illustrates a layout of a CMOS driver chip (MOSIS N84CAG) for a125 μm pitch 8×8 VCSEL array; and

FIG. 12 illustrates a SEM image of a cross section of a solder jointafter flip-chip bonding, according to embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. Overview of theInvention

Aluminum is the metal of choice for epitaxial layer connections andtop-layer bond pad metal in IC devices because of its superiorelectrical and thermal conductivity, low cost, wire-bondability, andother performance and reliability factors. However, aluminum is notsolder-wettable, because it oxidizes readily in air, and hence is notsuitable for flip-chip solder bonding. (Throughout this application, theterm aluminum is meant to include pure aluminum, and aluminum basedalloys.)

The present invention is an electroless plating process thatre-metallizes aluminum pads with gold. Gold is solder bondable andwire-bondable. The process works selectively on the aluminum pads only,without being detrimental to the underlying silicon circuitry that istypically protected by a passivation layer.

Electroless plating has the highest potential for cost reduction of thebond-pad coating processes. This occurs because the process provides aselective autocatalytic metal deposition directly on the aluminum padsof a CMOS chip, without requiring costly equipment forphotoresist-imaging or sputtering. Furthermore, electroless plating ismore cost effective than electroplating, because no external current isrequired. During the electroless process, metal is deposited by chemicalreduction reaction from an aqueous solution of the metallic salt, wherethe deposition solely depends on the autocatalytic action of the platingbath. Deposition progresses almost linearly with time. One can controlthe plating rate by controlling the temperature, concentration, and pHof the plating bath. Furthermore, electroless plating can be doneequally efficiently on a single semiconductor IC, or on a batchsemiconductor ICs.

Once the electroless process is complete, the gold plated bond pads aresolder wettable, and therefore can undergo solder bonding, includingflip-chip bonding.

During flip-chip bonding, electrical and mechanical interconnects areformed simultaneously by re-flowing the solder bumps at suitable atemperature and pressure. Flip chip bonding is a self-aligning process.The wetting action of the solder aligns the chip's bump pattern to thecorresponding solderable gold-coated substrate pads, therebycompensating for any chip-to-chip misalignment, up to a few microns.

An advantage of the invention is that the re-metallized aluminum/goldbond pads are still wire-bondable. In other words, the re-metallizedbond pads are suitable for both flip-chip solder bonding, and manualwire-bonding applications. Additionally, no masking or lithographicprocessing is utilized to protect the bond pads that are to bewire-bonded. This results in efficiency gains and cost reduction duringhybrid assembly.

2. Example Environment

Before describing the invention in detail, it is useful to describe anexample environment for the invention. Description of this exampleenvironment is provided for convenience only, and is not intended tolimit the invention in any way. In fact, after reading the inventiondescription, it will become apparent to a person skilled in the relevantarts how to implement the invention in alternate environments that aredifferent from that described herein.

Free space optical interconnects offer the following advantages for thecommunication of information: 3-D flexibility in packaging, highinterconnection densities, high bandwidth capability, independence ofinterconnection length, and immunity to electromagnetic interferenceduring the communication of information. FIG. 1 illustrates a free spaceoptical interconnect 101 having an optical transmitter 102, an opticalreceiver 114, and a compound lens 109. During operation, the transmitter102 sends information using laser light to the compound lens 108, whichfocuses the laser light on the receiver 114, thereby communicatinginformation.

In embodiments, the transmitter 102 includes a Vertical Cavity SurfaceEmitting Laser (or VCSEL) array 108 that is driven by an electronic CMOSdriver circuit 106. The array 108 and the CMOS driver circuit 106 aremounted on a pin grid array (PGA) 105, which is mounted on a PC board104. The array 108 includes arrays of optical switches that areintegrated with the corresponding CMOS logic circuitry 106. Thiscombination is referred to as a “smart pixel array” and is one of themost important building blocks in a free-space optical interconnectsystem. The receiver 114 includes a laser detector 110 that isintegrated with a CMOS receiver chip 111, for processing received laserlight. Similar to the transmitter, the detector 110 and CMOS receiver111 are mounted on a PGA 112, which is mounted on a PC board 116, asshown.

VCSELs have been identified as the optical source of choice in hybridsmart pixel applications because of their inherent two-dimensionalgeometry, which is suitable for high-density packaging. In one hybridapproach, the VCSEL array 108 is integrated with the CMOS driver chip106 utilizing flip-chip bonding, as shown in FIG. 2a. Additionally, thedetector 110 can be flip-chip bonded to the CMOS receiver 111 in asimilar manner.

Referring to FIG. 2a, the VCSEL array 108 has solder bumps 208 that areattached to VSCEL bond pads 204. The CMOS driver chip 106 hasaluminum-based bond pads 206. During flip-chip bonding, the VCSEL array108 is flipped upside down so that the solder bumps 208 are aligned withthe bond pads 206. Once sufficient alignment is achieved, the solderbumps 208 are re-flowed using the necessary temperature and pressureconditions to perfect the solder joint. Flip-chip bonding offers theadvantage of self-aligning properties of solder, as well as theelimination of parasitics that are associated with wire-bonding.

In order for a proper solder joint to be formed, the aluminum bond pads206 are re-metallized with gold prior to flip-chip bonding according tothe electroless plating process that is described herein.Re-metallization is necessary because the aluminum bonds that areattached to commercially available ICs are not solder wettable withoutfurther processing.

FIG. 2a also illustrates bond wire 202 having a ball bond 210 torepresent that the electroless plating process of the present inventionis still wire-bondable. In other words, the re-metallized aluminum pads206 that undergo the electroless plating process can still bewire-bonded after the process is complete. This assures flexibility andefficiency during post-plating hybrid assembly. Furthermore, there-metallized pads are wire-bondable without the use of any lithographicmasking techniques during plating, which can be expensive and timeconsuming.

FIG. 2b illustrates a partial cross section view of the various layersof the VSCEL array 108, the CMOS driver 106, and the solder bumps 208.Note that FIG. 2b is rotated 180 degrees relative to FIG. 2a, as theVCSEL array is on the bottom in FIG. 2b, instead of on top as in FIG.2a.

3. Description of the Electroless Plating Process According toEmbodiments of the Present Invention

As described above, semiconductor ICs from commercial foundriestypically have bond pads (also called electrodes or contacts) that aremade of an aluminum alloy, such as Al/Si/Cu (98%, 1%, 1%). Aluminumalloys are not solder-wettable, and therefore cannot be utilized inflip-chip bonding without re-metallization with a metal that issolder-wettable. The electroless process described herein builds a bondpad having layers of aluminum (original bond pad), zinc, nickel, andgold. The re-metallized bond pad is both solder-wettable andwire-bondable.

In the electro-deposition of metals (including electroless andelectro-plating), an aqueous solution is typically utilized because ofthe high solubility of most metal salts in water and the good electricalconductivity of such solutions. Metal deposition from metal saltsolutions is a chemical reduction reaction in both electrolytic andelectroless plating. However, electroless plating is a simpler and moreelegant process, because it depends solely on the autocatalytic actionof the plating bath, while electrolytic deposition is based on cathodicdischarge of metal ions and requires a external electric current.Electroless plating progresses almost linearly with time, resulting in avery uniform deposition, which is crucial for achieving uniform bumpheight over all the elements of a bond pad array. For bond padsmetallized with aluminum or sputtered aluminum alloy, the most robustre-metallizing system is nickel/gold. To initiate the electroless nickeldeposition, aluminum bond pads are activated with a seed layer of zinc.Next, a layer of nickel is added for hardness and mechanical strength.The final layer of metallization is the autocatalytic gold, whichensures wire-bondability as well as solderability of the bond padsurface. The entire re-metallization process flow can be summarized inthe flowchart 300 that is shown in FIG. 3, which will be referredthroughout sections 3.1-3.4 that follow.

3.1 Pre-Cleaning and De-oxidizing

Referring now to FIG. 3, in step 302, the bond pads are pre-cleaned withorganic solvents that include but are not limited to: TCA, Acetone, andMethanol. Other equivalent solvents could be used as will be understoodby those skilled in the arts. The bond pads need to be pre-cleaned inorder to remove dust and organic residues from wafer processing andtransport. Adhesion and homogeneity of electroless metal deposits sufferif the surface of the base metal is not sufficiently clean. Also, thepresence of contaminants increases the contact resistance appreciably.Hence, the entire IC is rinsed thoroughly in the mentioned organicsolvents, or their equivalents.

In step 304, the bond pads are de-oxidized to remove the native oxidelayer that is associated with aluminum. This step is necessary becauseif the native oxide layer is not removed completely from the aluminum,the subsequent zinc deposit becomes heavily nucleated and extremelynon-uniform. FIG. 4a is a Scanning Electron Microscope (SEM) image of azinc deposit without proper oxide removal. The zinc deposit in FIG. 4ahas distinct hexagonal crystals with poor overall surface coverage andvery high surface roughness (˜1.5 KÅ). In contrast, FIG. 4b illustratesa zinc layer deposited after proper oxide removal, and is discussedfurther in step 308 below.

In one embodiment, the oxide layer is removed by sputter-etching.

In an alternate embodiment, the oxide layer is removed by using a wetchemical etch. More specifically, the oxide layer is removed bymicro-etching the roughness of the aluminum bond pads to an approximaterange of 130-200 Å. A micro-etchant that can be utilized is an acidicliquid containing phosphoric acid (8% wt.), ammonium bi-fluoride (2%wt.), and butyl cellusolve (3% wt.). This micro-etchant combination iscommercially available under the trade name “The Stuff for Aluminum”,manufactured by Broco Products, Inc., and marketed by Seiler-Hughes ofBaltimore, Md. The miro-etchant is aggressive enough to penetratethrough the oxide barrier at room temperature (approximately 25 C). Themicro-etchant only reacts with exposed aluminum pads, without attackingthe SiO₂ passivation layer that protects the electronic circuitry.Therefore, the entire semiconductor IC can be treated with (or immersedin) the micro-etchant in an efficient manner, without utilizing anymasking techniques.

The dissolution rate of aluminum in the non-diluted micro-etchant isapproximately 0.1 μm/minute at room temperature. Therefore, given a bondpad having an initial thickness of 1.0 μm, the immersion time should notbe more than 1 minute. If the initial thickness of the aluminum layer isless, the solution is diluted to 50% volume/volume with de-ionized (DI)water, and the etching time is also reduced accordingly. By varying theconcentration and/or the etching time, the roughness of the resultingaluminum surface can be adjusted to the desired 130-200 Å range, whichimproves the adhesion and coverage of the subsequently applied zinclayer.

In step 306, the semiconductor IC (including the bond pads) is rinsed inde-ionized water to remove the micro-etchant, prior to the next step ofzincation.

3.2 Double Zincation

In step 308, a zinc layer is formed on top of the de-oxidized aluminumbond pads using an electroless process. This is a first zincation stepthat forms a seed layer for a second zincation step (step 312), andprevents the aluminum bond pads from getting re-oxidized. The zinc layeris formed by immersing the semiconductor IC (and bond pads) in analkaline zincate solution. The semiconductor IC should be immersed inthe zincate solution immediately after micro-etching and DI rinsing. Ifthe etched bond pads are exposed to air, they will reform a nativeoxide, preventing the proper formation of the zinc seed layer.

In embodiments, the zinc layer thickness is relatively thin, and isapproximately 0.25 um. However, other thicknesses could be used, as willbe understood by those skilled in the relevant arts.

In embodiments, a moderately alkaline plating solution, having theproduct name “Techni SBZ conditioner concentrate”, is utilized as thezincate solution. “Techni SBZ conditioner concentrate” is obtained fromTechnic, Inc. of Cranston, R.I., and has main ingredients of zinc oxide(5% wt.) and sodium hydroxide (30% wt.). A 10% zincate solution isprepared by adding 10 ml of SBZ concentrate to 90 ml of DI water. Theresulting pH of the solution is a approximately 8.5-9.0, becausedissolution rate of aluminum at this pH range is minimum.

Electroless zincation is a “displacement reaction”, in which aluminumions are displaced by zinc ions, as described in the reactions below:

Anodic dissolution of aluminum:

Al⁰+30H⁻=Al(OH)₃+3e⁻

Cathodic deposition of zinc:

Zn(OH)₄=Zn²⁺+4OH⁻

Zn²⁺+2e−=Zn⁰

2H⁺+2e⁻=H₂

Immersion time is a critical factor, because the alkaline zincate bathdisplaces the aluminum as mentioned above. Therefore if the immersiontime is too long, the whole aluminum bond pad can be displaced by thezincate solution. In embodiments, the immersion time for the firstzincation step is approximately 45 seconds at a temperature ofapproximately between 38-42 degrees C. In a preferred embodiment, thetemperature is 40 degrees C. However, other temperatures and immersiontimes could be used.

When done properly, zincation should produce a zinc film having a finegrain size of 0.1 um or smaller. Small grain size means better coverageand adhesion of the zinc layer and subsequent layers. Adhesion and grainsize of the zinc film can be influenced by traces of additives e.g.copper, iron, nickel, etc. in the zincate bath. Micro-etching (step 304)plays an important role in determining the grain size and uniformity ofthe grain distribution of the zinc deposit. FIG. 4b shows the morphologyof a zinc layer with proper micro-etching. The zinc film in FIG. 4b hasa vastly improved surface coverage and finer grain size (0.1 μm), whencompared to the zinc deposit in FIG. 4a.

Regarding the zincation solution, it is noted that Techni, Inc. (themanufacturer of Techni SBZ) recommends that zincation be done at roomtemperature (25 C). However, by experiment, the inventors found that thetemperature range of 38-42 degrees produced a better fine grain zincfilm on the aluminum alloy bond pad, as shown in FIG. 4b. Outside of the38-42 degree temperature range, the grain size increases more towardthat shown in FIG. 4a.

In step 310, the bond pads are de-smutted. More specifically, thesemiconductor IC (and bond pads) are immersed in an aqueous solution ofapproximately 20% nitric acid at room temperature (25 C) forapproximately 15 seconds. The nitric acid solution dissolves some of thefirst zinc layer in preparation for a second zincation treatment.

In step 312, the semiconductor IC (and bond pads) is rinsed withde-ionized water, prior to the second zincation step.

In step 314, a second zincation is performed to rebuild the zinc layer(after de-smutting) using the same type of zincation solution as in step310. In embodiments, the zinc layer is built up to approximately 0.25um. Double zincation is preferred over single zincation, because theintermediate “de-smutting” in nitric acid strips the granulated initialzinc deposit, resulting in a more uniform zinc film.

When “Techni SBZ” is used as the zincation solution, then the secondzincation immersion is for approximately 30 seconds at the mentionedtemperature range of approximately between 38-42 degrees C. As mentionedabove, in a preferred embodiment, the temperature of the zincatesolution is 40 degrees C.

In step 316, the semiconductor IC (and bonds pads) is rinsed withde-ionized water.

3.3 Electroless Nickel Plating

In step 318, a nickel layer is deposited on top of the zinc layer usingan electroless process. The nickel layer seals the aluminum surface as asolder-diffusion barrier layer, and also provides hardness, mechanicalstrength and solderability to the bond pads.

Typically, two types of autocatalytic nickel plating systems aresupplied by commercial vendors, based on either nickel-phosphorus (NiP)or nickel-boron (NiB). The reducing agent in Ni/P system is sodiumhypophosphite, while the activator in Ni/B system is dimethyl aminborane (DMAB). In both cases, the deposit is Ni/P or Ni/B alloy ratherthan pure nickel. Additionally, in recent years it has been found thatpure nickel deposits can be formed using gaseous hydrazine as reducingagent, though no significant practical application has been reportedyet. Each of the these techniques of nickel deposition can be utilizedby the present invention for nickel deposition on the zinc layer.However, the NiP based deposition is preferred because the hypophosphitebased plating solutions are well-known to produce good quality ofelectroless nickel with a high plating rate (>0.25 um/min).

In all the aqueous electroless solutions, nickel ions (nickel chlorideor nickel sulfate) and reducing agents remain in metastable equilibrium.In addition, most of the standard aqueous nickel plating bathformulations contain organic complexants (citrate or acetate), pHregulators. accelerators, stabilizers, buffers and wetting agents toinhibit the occurrence of random reduction of nickel and thereby, toimprove adhesion and morphology of the deposit.

There are different theories to explain the deposition mechanism of theNi/P alloy, viz. catalytic model involving atomic hydrogen, hydratedanion model, hypophosphite ion adsorption model etc. Essentially, nickelis deposited initially by ion-exchange or displacement reaction aided bythe reducing agent. The chemical reaction is as follows:

Zn⁰+Ni(H₂O)₆ ²⁺=Zn(H₂O)₆ ²⁺+Ni⁰

Once initiated, the process autocatalyzes itself to continue nickeldeposition at a more or less linear rate depending on the temperatureand pH of the solution.

The Ni plating rate is ˜0.3 μm/min at 90° C. on zincated aluminum usinga high-phosphorus (11%) electroless nickel solution with pH adjusted to4.5. The reagent that is utilized is a commercially obtainableelectroless nickel plating solution, named Techni EN 9120, manufacturedby Technic, Inc of Cranston, R.I. However, other regents could be usedas will be understood by those skilled in the arts based on thediscussion herein. Additionally, the temperature range for nickelplating can be performed over a range of 85-91° C., with a preferredtemperature of 90 degrees C.

Techni EN 9120 is provided in two separate parts, part A and part B,which are to be combined and diluted with de-ionized water prior totheir use. Part A is the source of nickel ions, such as nickel chlorideand nickel sulfate. Part B is the source of hypophosphite ions, such assodium hypophosphite. The concentration of nickel and hypophosphite inthe solutions is 0.65-0.85 oz/gal and 3.2-4-5 oz/gal, respectively. Theplating bath is prepared by mixing 5 ml of part A, and 15 ml of part B,with 80 ml of DI water. The optional additional step of using an“alkaline strike” prior to the nickel deposition is discarded, as itdoes not significantly improve the quality of the metallic deposit.

FIG. 5 illustrates an SEM photograph of a bond pad bumped with a 3.0 μmhigh nickel barrier using the nickel plating process described herein.Roughness of the bump surface is approximately 450 Å. Other nickel layerthicknesses could be formed and utilized, as will be understood by thoseskilled in the arts based on the discussion herein.

In step 320, the semiconductor IC (and bond pads) are rinsed inde-ionized water.

3.4 Immersion Gold Plating

In step 322, a layer of gold is formed on the nickel layer using animmersion process. The gold plating protects the re-metallized bond padsfrom oxidation and also increases solderability and wire-bondability.

In embodiments, an alkaline (pH 8.9) immersion gold bath of sodium goldsulfite is utilized, having a gold concentration of 4.1 g/liter. Thissolution plates a thin layer of gold at a rate of 0.02 μm/min at 70° C.by partial displacement of the nickel layer. The mentioned solution isavailable commercially under the product name Oromerse SO, from Technic,Inc., of Cranston, R.I.

Oromerse SO comes in two parts (A and B), which are mixed togetherbefore use. To prepare a 1000 ml solution, 934 ml of part A is mixedwith 66 ml of part B. The solution is not diluted further with water.

The immersion gold layer acts as a seed layer for a subsequentautocatalytic gold deposition. Therefore, a thickness of 0.1-0.2 μm issufficient. The maximum thickness achievable by immersion gold platingbefore the displacement reaction stops is 0.5 μm, which is enough forwire-bonding.

FIG. 6 is an SEM photograph of immersion gold plated bond pad. FIG. 6reveals that the texture of the immersion gold is almost exactly similarto that of the underlying nickel surface. However, the surface roughnessrange is wider at (350-700 Å).

In step 324, the semiconductor IC (and bond pads) are rinsed inde-ionized water.

3.5 Autocatalytic Gold Plating

In step 326, the thickness of the gold layer is increased using anautocatalytic gold plating process.

Immersion gold plating is often sufficient for flip-chip andwire-bonding. However, sometimes it is recommended to increase thethickness of the gold layer for ease of wire-bonding. For that, we usean additional re-metallization step, called autocatalytic gold plating,which ensures wire-bondability of the peripheral bond pads whilemaintaining the solderability of the bond pad array.

Autocatalytic plating is a relatively slow process (e.g. deposition rateof 1.0 μm/hour). Autocatalytic plating is sensitive to temperature, loadfactor of the plating bath, agitation, and the prior condition of thesurface to be plated. If the immersion gold surface is oxidized, thesubsequent autocatalytic gold deposit suffers from exfoliation orpeeling.

In embodiments, a pH neutral non-cyanide based solution is utilized forthe autocatalytic gold plating solution. More specifically, Neorum TWBis utilized as the autocatalytic gold plating solution, which isobtainable from Uyemura International Corporation, Ontario, Canada.Other solutions could be utilized as will be understood by those skilledin the arts based on the discussion herein.

Neorum TWB comes in three separate parts, namely: Neorum TWB-1M, NeorumTWB-1S, and Neorum gold solution. A 1.0 liter plating bath is preparedby mixing together the following: 915 ml of Neorum TWB-1M, 5 ml ofNeorum TWB-1S, 51.4 ml of Neorum gold, and 28.6 ml of de-ionized water.The combined solution has sodium sulfite in it. The sodium sulfitereduces metallic gold from the electroless bath, which contains organicand inorganic salts of gold (gold concentration 3.5-4.5 g/lit), alongwith additives and complexing agents. In embodiments, a 1.0 μm thicklayer of autocatalytic gold encapsulates the entire bond pad giving it amushroom shape rather than a rectangular shape with vertical side-walls.The roughness of the surface after the final coating of autocatalyticgold is 250-350 Å.

FIG. 7 illustrates the resulting re-metallized bond pad 700 aftercompleting the re-metallization process 300. The re-metallized bond pad700 includes: the original aluminum bond pad 702 on a silicon substrate708, a zinc layer 714 on top of the aluminum bond pad 702, anelectroless nickel layer 712 on top of the zinc layer 714, an immersiongold layer 706 on top of the electroless nickel layer 712, and anautocatalytic gold layer 710 on top of the immersion gold layer 706. Thelayer thicknesses that are illustrated in FIG. 7 are for examplepurposes only and are not meant to be limiting. Other layer thicknesscould be chosen without changing the scope and spirit of the invention,as will be understood by those skilled in the relevant arts based on thediscussion herein.

The re-metallizing process 300 is a low-cost technique forre-metallizing aluminum bond pads based on electroless plating. Theprocess is low cost because the electroless plating solutions that arementioned in the process are readily available in the commercial market.Additionally, the number of steps in the process cycle has been reducedby selecting an advantageous combination of chemical reagents for thesteps. Potentially hazardous steps, like cyanide zinc pre-treatment andcyanide autocatalytic gold plating have been replaced by benignalternative processes.

As discussed herein, the re-metallization process 300 is relevant to theprocessing and packaging of electronic and optoelectronic devices.However, the process 300 is not limited to these applications. There-metallization process can also be employed for solder attachment offibers to waveguides, and the fabrication of under-bump-metallurgy forsolder bumps. Additionally, the process is relevant to applications invarious other industries such as: aerospace, automobiles, portablecommunication systems etc. Also, the re-metallization scheme foraluminum can be extended to other systems of electroless metaldeposition, e.g. platinum, palladium, etc. These other process andapplications will be apparent to those skilled in the arts based in thediscussion herein, and are within the scope and spirit of the invention.

4.0 Variation in Surface Roughness

The roughness of the bond pad surface at the different stages of there-metallization process has been measured. The adhesion of a metallayer to its preceding layer is strongly influenced by the roughness ofthe surface. Since re-metallization involves multiple metal layers,adhesion strength at each interface has a significant contribution tothe overall robustness of the re-metallized bond pad. The histogram inFIG. 8 depicts the variation of roughness for chemically pure aluminumand Al/Si/Cu alloy.

5.0 Size and Pitch of the Re-Metallized Bond Pads

The re-metallized bond pads expand laterally during the re-metallizationprocess in addition to increasing in thickness. Therefore, there-metallized bond pads have a mushroom shape, as illustrated in FIG. 9.The mushroom shape is attributable to the absence of any photoresistmask during the electroless deposition processes. Because of the lateralexpansion, the pitch between the individual array elements becomes animportant factor. To prevent short-circuiting of two adjacent pads, thegap between them should be greater than twice the verticalre-metallization height. In the example of FIG. 9, the thickness of themetal deposit is 5.0 μm, and the array pitch is 125 μm. Each flip-chipbond pad on a MOSIS chip NF8Y-AF is approximately 70×70 μm². Hence, thelateral gap between two adjacent pads is (125-70)=55 μm, which is wellabove the maximum lateral growth of 10 μm.

It has been experimentally verified that if there is sufficient gap forallowing lateral spreading, then the thickness obtained from electrolessprocess is independent of the individual size and shape of the bond padsfor dimensions on the order of 50 μm or larger. However, when the sameprocess was applied on circular bond pads with 20 μm diameter, uniformbump height over the whole area array was achieved, but the thickness ofthe metallization was less than what is expected for 50 μm or largerbond pads. Also, it has been observed that the coating of the groundpads improves significantly if the backside of the CMOS chip isinsulated.

6.0 Reproducibility of the Process

The re-metallization has been successfully tested on e-beam depositedpure aluminum and sputtered aluminum alloy without any modification tothe process cycle. The yield was approximately 100% on the sputteredaluminum alloy (as used MOSIS chips). As mentioned, the testing was alsosuccessful for the pure aluminum. Hence, the process is universallyapplicable to all kinds of aluminum substrates.

7.0. Hybrid Packaging: Experiments and Results

As mentioned herein, the semiconductor ICs with the re-metallized bondpads are likely to be assembled into hybrid packages with otherelectronic and/or optoelectronic components. In the sections 7.1-7.4,tests and configurations related to the hybrid assembly are discussed,including wire-bonding and ball shear testing, and flip-chip bonding.

7.1 Wire-bonding and Ball Shear Test

Gold wire-bonds on a gold-coated surface is the best condition forreliable wire-bonding. Wire-bondability of the re-metallized bond padswas tested on many samples and the resulting yield was 100%. Ball bondswere made on the re-metallized surface using a 0.8 mil(˜20.0 μm) goldwire at a fixed stage temperature of 135° C. The average destructiveball shear force measured by a DAGE2400 shear tester was approximately100 gm. The shear occurred along the interface between the base of thegold ball and the autocatalytic gold layer, which proves that thedeposited metal layers are well-adherent to each other.

7.2 Flip-Chip Bonding

A VCSEL device with solder bumps was flip-chip bonded to a re-metallizedCMOS driver chip (as shown in FIG. 2), using a commercial flip-chipaligner/bonder M-8A (from Research Devices, NJ). With propercalibration, the alignment accuracy achievable by the M-8A machine isbetter than 2.0 μm.

The “tack and reflow” technique for solder bonding was used and isdescribed as follows. After the VCSEL die and CMOS substrate areoptically aligned, the solder bumps are tacked to the re-metallized bondpads with simultaneous application of temperature (below the meltingpoint of the solder) and pressure. After tacking, the solder is reflowedat a temperature above its melting point to form mechanical andelectrical interconnections between the two chips. This technique takesadvantage of the self-aligning property of the molten solder.

In embodiments, ductile solder (50 wt % indium-50 wt % lead alloy)material was used to reduce the influence of shear strain that is causedby mismatches between the thermal expansion of the materials. Indiumlead solder has an eutectic melting point of 207° C. Therefore, tackingwas carried out at 190° C. with 6.0 MPa of pressure. Reflowing wascarried out at 227° C. for 1 minute, and then cooled down gradually toroom temperature. The temperature profile used during the flip-chipsolder process is shown in FIG. 10.

A daisy-chained configuration of solder bumps and bond pad array wasused to verify the continuity of the electrical path after flip-chipbonding. The resistance between two electrically isolated bond pads,which where connected only by a flip-chip bonded solder bump, wasmeasured with a multimeter. If an open circuit was measured, it wouldmean that the bonding process had failed to connect the two bond pads onthe CMOS chip. No open circuits were found. The average resistance persolder joint was 0.39 ohm.

FIG. 11 illustrates the layout of the 8×8 CMOS driver chip N84CAG. AVCSEL die with a similar bond pattern has been successfully flip-chipbonded onto the CMOS driver chip. This flip-chip bonded pair has beenmodularized in a PGA package and successfully tested for laser emission,which proves the feasibility of the electroless re-metallization processand flip-chip bonding for hybrid integration.

Additionally, the electroless plating is a scalable process, as it canbe applied on bigger arrays of bond pads, including 16×16 and 24×24 bondpad arrays.

7.3 Cross Section of the Solder Joint

FIG. 12 illustrates an SEM image 1200 of a solder joint produced byflip-chip bonding a re-metallized bond pad. Image 1200 illustrates thedifferent metal layers resulting from re-metallization and solderbonding including: an under-bump metal 1202, an indium lead solder bumplayer 1204, a molten solder layer 1206, a gold layer 1208, nickelbarrier layer 1210, a zincated aluminum pad layer 1212, and a siliconsubstrate layer 1214.

Image 1200 clearly shows that the molten solder layer 1206 has wettedthe metal deposit on the aluminum bond pad 1212. This illustrates thatthe aluminum bond pad is successfully re-metallized into a solderablegold surface by the electroless process described herein. Additionally,since the seed layer of zinc is ultra-thin and mostly displaced bysubsequent nickel layer 1210, it is not distinguishable in the picture.However, the nickel barrier layer 1210 is easily recognized.

7.4 Packaging of the Hybrid Pair

As further proof concept for hybrid assembly, a CMOS substrate with aVCSEL die flip-chip bonded on top of it was attached to a 14-pinbutterfly package with conductive silver-epoxy. The peripheral. bondpads on the CMOS chip were connected to contact pads on the butterflypackage by wire-bonding.

CONCLUSION

Example embodiments of the methods and components of the presentinvention have been described herein. As noted elsewhere, these exampleembodiments have been described for illustrative purposes only, and arenot limiting. Other embodiments are possible and are covered by theinvention. Such other embodiments will be apparent to persons skilled inthe relevant art(s) based on the teachings contained herein. Thus, thebreadth and scope of the present invention should not be limited by anyof the above-described exemplary embodiments, but should be defined onlyin accordance with the following claims and their equivalents.

What is claimed is:
 1. A re-metallized aluminum-based contact on asemiconductor (IC), comprising: an aluminum layer; a zinc layer that isadjacent a top surface of said aluminum layer, wherein said zinc layerhas a maximum grain size of approximately 0.1 um; a nickel layer that isadjacent a top surface of said zinc layer; and a gold layer that isadjacent a top surface of said nickel layer.
 2. The re-metallizedaluminum-based contact of claim 1, wherein said zinc layer is depositedusing a zincate solution having an approximate temperature of 38-42degrees C.
 3. The re-metallized aluminum-based contact of claim 1,wherein said zinc layer is deposited using a zincate solution having anapproximate temperature of 40 degrees C.